The present invention relates to the use of certain lanthanide compounds, such as cerium and lanthanum oxides, carbonates, or phosphates, as strength enhancers for calcium aluminate refractory concretes, mortars, and brick products having a minimum of 40% alumina, and especially for improving the strength of alumina refractory products in the 500 to 1000 degree Fahrenheit range. Refractory products are useful in insulating portland cement concrete in jet engine test cells. Large portland cement concrete buildings are used as jet engine test cells to evaluate aircraft engines indoors so that surrounding communities will be spared from noise and smoke pollution. The problem with these engine test structures is that portland cement concrete erodes and spalls above 500 degrees Fahrenheit. The air temperature in jet engine test cells, however, has been recorded in the range of 400 to 900 degrees Fahrenheit. The resulting deterioration of the jet engine test cells thus causes: expensive shut down time for repairs; expense for the repairs; and, possible damage to expensive aircraft jet engines from concrete debris. Debris from spalling concrete can easily be swept into the air intakes of jet engines and thus seriously damage the engines.
Research has been underway to determine whether refractory mortar could be used to insulate portland cement concrete from heat. Refractory mortar is a high alumina cement mixed with bauxite or chrome ore aggregate and water. It is placed or gunned (sprayed) on as with conventional portland cement concrete products, but is then slowly heated to 2000 to 3000 degrees Fahrenheit to produce a material with ceramic like properties.
During the heating process, the refractory mortar loses a considerable amount of strength as it goes through several hydrated and dehydrated phases before reaching the ceramic phase. Unfortunately, the weakest area during the heating process is from 500 to 1000 degrees Fahrenheit, in the temperature range that has been recorded for jet engine test cell concrete surfaces during use. Thus, it is highly desirable to have a means to increase the strength of portland cement concrete in this temperature region of from 500 to 1000 degrees Fahrenheit to provide an improved refractory material.
The present invention differs from that disclosed in the aforementioned U.S. Pat. No. 4,661,160 primarily in that the present invention is for lanthanide elements. The lanthanide elements as described in Advanced Inorganic Chemistry, A Comprehensive Text by F. Albert Cotton and G. Wilkinson (Interscience Publishers, John Wiley & Sons, 1962) at chapter 31, page 870 are strictly the 14 elements following lanthanum in which the fourteen 4f electrons are being successively added to the lanthanum configuration. These fourteen elements are cerium, dysprosium, erbium, europium, gadolinium, holmium, lutetium, neodymium, praseodymium, promethium, samarium, terbium, thulium, and ytterbium. The prior art discloses alkaline earth elements, which have smaller "p" orbitals (f&gt;d&gt;p&gt;s in size comparison). The lanthanide elements of the present invention all have high oxidation states of +3 and +4 [ex: Ce.sup.+3, Ce.sup.+4, the Ce(IV) being seen in CeO] whereas the prior art patent is limited to the +2 valence (Ba.sup.+2, Sr.sup.+2, Ca.sup.+2, Mg.sup.+2). The lanthanide elements form relatively water insoluble oxides (ex: CeO.sub.2 is insoluble) and these oxides do not react with water to give alkaline hydroxides (pH8 or greater). In contrast, all the elements from the prior art patent have water soluble, highly reactive, alkaline oxides, which react with water to yield hydroxides [ex: BaO in water yields Ba(OH).sub.2 and SrO in water yields Sr(OH).sub.2 ].
Because of the differences in chemistry, it has been observed that the lanthanide compounds, particularly the oxides (ex: CeO.sub.2), substitute for alumina (Al.sub.2 O.sub.3) in the mortar mix; whereas, the alkaline earth compounds (ex: BaO or SrO) substitute for the lime (CaO) or calcium oxide in the mortar mix. This means that the formations of increased strength are due to two different mechanisms: the present invention in which the cerium oxide or lanthanum oxide replaces the alumina or aluminum oxide, and the prior invention of U.S. Pat. No. 4,661,160 in which the barium oxide or strontium oxide substitutes for the calcium oxide.
An advantage of the lanthanide compounds over the alkaline earth compounds is that the lanthanide oxides, phosphates, and carbonates are unreactive at room temperature and can be added directly to wet mortar or concrete mix without the development of heat (exothermic reaction) or premature flash setting (rapid hardening).
The present invention also differs from U.S. Pat. No. 4,406,699 which discloses the use of yttria additions of about 8 to 15 percent by weight as a stabilizing oxide to eliminate or substantially minimize the nonuniform expansion of zirconium oxide. U.S. Pat. No. 4,406,699 also mentions the use of the lanthanide series oxides as a substitute for yttria. U.S. Pat. No. 4,406,699 does not provide test results indicating that the lanthanide series oxides were actually tested and evaluated.
Tests were performed on refractory concrete products using yttrium oxide (yttria, Y.sub.2 O.sub.3) and zirconium oxide (zirconia, ZrO.sub.2) as disclosed by U.S. Pat. No. 4,406,609, as well as lanthanum oxide and phosphate and cerium oxide, phosphate and carbonate. These tests demonstrated that yttria and zirconia do not exhibit the same strength enhancement characteristics as the lanthanum oxides, phosphates, and carbonates. The strength enhancement characteristics of the lanthanide compounds were not disclosed by U.S. Pat. No. 4,406,609 and these strength enhancement characteristics were discovered only after the actual testing of the lanthanide compounds.
While yttrium is often associated with the ores of the lanthanide elements, yttrium is not in the lanthanide family according to Advanced Inorganic Chemistry, A Comprehensive Text.